(1) Field of the Invention
The present invention relates to an arrangement for increasing the precision of a machine tool and finds particular utility in relation to a machine tool of large dimensions.
(2) Description of the Prior Art
In machine tools, in order to obtain workpieces within the tight dimensional tolerances that are nowadays required, the position assumed by the tool relative to the workpiece as a result of displacements of the various carriages of the machine tool along their respective coordinate axes, must coincide with the desired position of the tool.
In known machine tools currently in use, the position assumed by the tool relative to the workpiece deviates from the desired position due to a number of factors prime among which is the elastic deformation of parts of the machine tool. The magnitude of this deviation is dependent on the values of various parameters characterizing the current working state of the machine tool (such parameters include the coordinate positions of the carriages).
Another cause of such deviations lies in thermal deformations in the structure of the machine tool arising from uneven heating and/or cooling of its parts; the magnitude of such deformations will depend on the temperatures of various points of the machine tool itself. A further cause lies in wear of the tool, so that its operative point is not in the desired position.
Yet another cause of undesired deviations between the desired and actual positions of the tool, is that during assembly the machine tool structure may experience geometric errors due to misalignments and errors in perpendicularity for example.
These deviations, which in general have linear components and angular components, are particularly significant in large machine tools and result in the production of workpieces with faults in parallelism and perpendicularity.
Various arrangements have been suggested to prevent such deviations and include auxiliary stiffening structures, mobile counterweights and/or mechanical compensator devices responsive to a control signal generated by members of the machine tool itself, thermal conditioning systems associated with the machine tool itself and/or the environment in which the machine tool is installed, and adjustment devices providing predetermined adjustments to the parallelism and perpendicularity between the axes of carriage movement.
These arrangements are not entirely satisfactory in that they act to reduce the deviations by removing individually only the particular cause that is possible for the arrangement to remove. They also generally have the disadvantage of being complicated and result in a considerable increase in the size and the cast of the machine tool.
It is therefore an object of the present invention to provide an improved arrangement for increasing the precision of a machine tool.